Depression is a mood altering condition, the closest comparison to it at it’s suicidal stage would be a person suffering from delirium during a physical sickness. Perhaps even the emotional crying jag a person who is real drunk can go on.
We try to talk them out of suicide because we are human and compassionate towards each other, though, admittedly, not all of us. We understand that their condition will improve with treatment, just as the crying drunk will be better when he/she sobers up, and that the suicidal urges are a symptom of the illness, therefore inappropriate.
It doesn’t matter that what they tell us as reasons justifying their suicidal intentions might be really good, because the natural instinct of any normal human being under reasonable conditions is to survive. To wish to die while still reasonably physically healthy is not normal, therefore it is a temporary symptom of an illness.
We will patiently care for a person suffering from any number of physical illnesses that they can recover from, even if in delusion they wish to die, because we understand that the wish is not real, it is part of the pneumonia, the flue, small pox or diphtheria. We will work to keep them alive and healing.
The same goes with depression, only it is more insidious, because most depressed people seem just fine. They do not look sick. Plus, there is a form of depression known as Smiling Depression, where the person appears outwardly happy, but in reality is inwardly miserable. It sometimes is problematic for another to realize just how depressed a suicidal person can be.
A person discussing suicide is usually acting on an instinctive, automatic impulse to get help, without actually knowing it most of the time. A determined suicidal person often will decide what to do, perform a certain ritual, then go and carry it out without telling anyone.
Therefore, if your friend discusses suicide with you, he is seeing, without knowing it, a reason to live. That does not indicate that he is not going to kill himself though. Depressives can discuss suicide a few times, then go out and do it if the depression is deep.
If he is not under therapy, get him to a therapist and onto antidepressants fast! Run him through the local ER if necessary.